After 2014, information dries up because, as the Revenue explained in response to a freedom of information request in 2016, "by disclosing numbers of enquiries... HMRC is at risk of inadvertently providing people with the means of identifying coverage rates".

What triggers investigations?

While the Revenue doesn't publish a big list of behaviours it looks out for, there are certain red flags.

High variation in turnover or profit from year to year is one example. Consistently paying little or no tax is another.

If your business is substantial and profitable but you are still managing your own accounts rather than working with an accountant, that can also raise suspicions.

Anything that seems out of the ordinary or out of step with the norm for your sector or region will at the very least prompt further scrutiny.

Supercomputers and whistleblowers

Since 2010, HMRC has used a powerful data analysis system called Connect to pull in and cross-reference vast amounts of information on taxpayers.

It uses data from other government agencies, such as the Driver and Vehicle Licensing Agency (DVLA) as well as online auction and car trading websites, to identify anomalies.

Collating all that data may highlight someone reporting minimal profit from a business in a generally profitable sector, while at the same time whizzing about in flashy cars.

On the other hand, many investigations are the result of something less high-tech: tip-offs from disgruntled neighbours, friends, relatives or former employees.

In 2017/18, HMRC's tax fraud hotline received 40,000 calls - and informants netted more than £340,000.